1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, and a process cartridge and an electrophotographic apparatus which have the electrophotographic photosensitive member.
2. Related Background Art
In recent years, for the purpose of overcoming disadvantages of inorganic electrophotographic photosensitive members making use of inorganic photoconductive materials, organic electrophotographic photosensitive members making use of organic photoconductive materials have been energetically put forward. In order to satisfy both electrical properties and mechanical properties, organic electrophotographic photosensitive members are often provided with a photosensitive layer which is a multi layer type (function separated type) photosensitive layer in which a charge generation layer containing a charge generating material and a charge transport layer containing a charge transporting material are superposingly formed.
Electrophotographic photosensitive members are required to have a stated sensitivity, electrical properties and optical properties which have been adapted to electrophotographic processes applied. Electrophotographic photosensitive members are also required to have durability to electrical and mechanical external forces because such forces are directly applied thereto through corona charging or contact charging, imagewise exposure, development by toner, image transfer, surface cleaning and so forth.
As to methods for improving wear resistance of the surfaces of organic electrophotographic photosensitive members, there is known in the art a method in which the binder resin of a surface layer is made to have a high molecular weight, a method in which a filler is added to the binder resin of a surface layer, a method in which the structure of a binder resin is incorporated with a siloxane structure of a structure for imparting lubricity (slipperiness) such as a fluorine-containing substituent or a solid lubricant such as polytetrafluoroethylene (PTFE) is added so as to reduce the coefficient of friction with cleaning means such as a cleaning blade.
As another method for improving wear resistance of the surfaces of organic electrophotographic photosensitive members, it is proposed to use a binder resin having good mechanical strength.
However, even if the binder resin itself has good mechanical strength, its use in mixture with a low-molecular-weight charge-transporting material can not sufficiently make the most of the mechanical strength the binder resin has originally, and such a binder resin has not necessarily achieved satisfactory durability (wear resistance or scratch resistance). On the other hand, if the charge-transporting material is added in a smaller quantity in an attempt to make the most of the mechanical strength the binder resin has originally, a problem may arise such that it causes a lowering of electrophotographic sensitivity or a rise of residual potential. That is, this proposal has not achieved both the surface mechanical strength and the electrophotographic performance.
The method in which lubricity is imparted to the surfaces of electrophotographic photosensitive members to reduce the coefficient of friction with cleaning means such as a cleaning blade may also cause a lowering of the surface mechanical strength, and has not succeeded in achievement of sufficient durability.
The use of a high molecular weight charge transporting material for the purpose of better preventing the electrophotographic photosensitive member surface from having a low mechanical strength because of the addition of a low molecular weight charge transporting material is disclosed in Japanese Patent Applications Laid open No. 64-9964, No. 2-282263, No. 3-221522, No. 8-208820 and so forth. In many of these, however, it is not necessarily the case that the surfaces have sufficient wear resistance. Even in those having a mechanical strength to a certain extent, there has been a disadvantage that the manufacturing cost is too high to be suited for practical use.